(i) Field of the Invention
The present invention relates generally to grain drying equipment. In particular, the present invention relates to a heat exchanger that utilizes the moist hot exhaust air from a grain dryer to pre-heat ambient air prior to entering the blower of the grain dryer.
(ii) Description of the Related Art
Grain dryers are generally well known in the art. A typical prior art grain dryer is shown in FIG. 1. which is a cut-away view of a single blower and single plenum grain dryer. Most grain dryers in use today utilize this design or variations on it.
Grain dryers of the type shown in FIG. 1 are known in the prior art and therefore its construction and operation will only be generally described herein. The typical grain dryer is constructed from steel and sheet metal. The major components of a typical grain dryer 11 include: a blower 10, a top conveying auger 12, grain columns 14, an air plenum 16, inner porous walls 18, outer porous walls 20, a lower conveying auger 22, and an air heater and an air mixing chamber 24.
A typical grain dryer 11 operates by receiving a supply of wet or moist grain from a separate conveyor (not shown) at a top opening of the dryer and transporting the wet or moist grain across the top of the grain dryer 11 via the top conveying auger 12. The grain then flows by gravity downward from the top conveying auger 12, down through the grain columns 14 where it is dried and then to the lower conveying auger 22. The dried grain is then transported via the lower conveying auger 22 to a bottom opening of the dryer and out of the dryer 11.
The grain is dried in the grain dryer 11 via the flow of hot dry air through the grain columns 14. The grain columns 14 are separated from the air plenum 16 by the inner porous walls 18, are separated from the exterior environment of the grain dryer 11 by the outer porous walls 20, and are separated from other adjacent grain columns 14 by inner panels 26. Ambient air is drawn directly from the exterior environment of the dryer and into the grain dryer 11 via the blower 10. The blower 10 blows the ambient air through a heating chamber 24 where the ambient air is heated as it flows through the heating chamber. The heated air is then blown through an air mixing chamber 24 to ensure that the air is evenly heated. The thoroughly heated and mixed air hen flows into the air plenum 16. The action of the lower 10 blowing air into the air plenum 16 causes the air pressure in the air plenum 16 to rise to a pressure higher than that of the atmospheric pressure of the exterior environment of the grain dryer 11. The higher pressure in the air plenum 16 causes the heated air to flow through the inner porous walls 18 of the grain columns and into the columns 14. The heated air then flows through the grain that is moving downwardly through the grain columns 14 and then out through the outer porous walls 20 to the exterior environment of the grain dryer 11. When the heated air flows through the grain passing through the grain columns 14, moisture is extracted from the grain by the heated air and is conveyed to the exterior environment of the grain dryer 11. Therefore, as the grain flows downwardly through the grain columns 14 and hot air is passed from the air plenum through the inner porous walls 18, around the grain and through the grain columns 14 and out through the outer porous walls 20 to the exterior environment of the grain dryer 11, the grain is heated and moisture is removed. The desired moisture content of the grain exiting the grain dryer 11 via the lower conveying auger 22 can be controlled by altering the flow rate of grain leaving the grain dryer 11 which controls the rate that grain passes downwardly through the grain columns or by changing the temperature of the heated air being blown into the air plenum 16.
A disadvantage of this typical grain dryer 11 is that the moist hot exhaust air leaving the grain columns 14 via the outer porous walls 20 exits to the exterior environment of the grain dryer 11 while it is still at a temperature substantially above that of the ambient air being drawn into the grain dryer 11. Therefore, significant amounts of energy can be wasted by the exhausting to the exterior environment of the grain dryer 11 moist hot air that is still capable of heating grain and possibly absorbing additional moisture.
Another type of grain dryer is shown in the Noyes et al. U.S. Pat. No. 4,268,971. This grain dryer has both a heating and a cooling mode. This is accomplished by dividing the typical single air plenum into two vertically separated chambers. The grain is heated and moisture is removed in the upper half of the dryer by blowing heated air through the upper chamber of the plenum, through the inner porous walls of the upper half of the grain dryer, around the grain and through the grain columns and out through the outer porous walls of the upper half of the dryer. The grain is cooled in the lower portions of the grain columns by the blower drawing ambient air through the outer porous walls of the lower half of the grain dryer, around the hot grain and through the grain column and through the inner porous walls of the lower half of the grain dryer and into the lower chamber of the air plenum. This cooling air, which is now partially heated and containing moisture removed from the grain, is then drawn from the lower chamber of the air plenum into the blower and mixed with ambient intake air being drawn into the grain dryer to be heated and used to dry grain. This grain dryer also includes a structure for recycling the moist hot exhaust air exiting the grain drying section of the grain dryer along with partially heated air from grain cooling. The moist hot exhaust air is directed to the intake of the blower. The moist hot exhaust air is mixed with the ambient air and with the partially heated air from the second chamber of the air plenum that was utilized to cool the grain. Thereby, the air entering the heater is significantly warmer than that of the ambient air and requires less energy input from the heater section to effectuate the drying of the grain. A significant disadvantage of this grain dryer is that the moist hot exhaust air that is being mixed with the ambient air and with the partially heated air from the lower chamber of the air plenum has a high moisture content. The high moisture content is a direct result of the heating of and moisture extraction from the grain. Since moist hot exhaust air already has a higher moisture content, its additional moisture holding capability is less than that of the cooler and dryer ambient air. Likewise, the partially heated air from grain cooling also has an elevated moisture content above that of the ambient air. Therefore, while requiring less energy input to obtain a desired air temperature, the moisture removing capability of the heated air being used to dry grain is reduced. Therefore, the grain flow rate through the dryer will need to be reduced or the amount of air flowing through the grain dryer will need to be increased to compensate for the reduced moisture holding capacity of the heated air resulting in limited overall savings of energy or time, and possibly reducing grain throughput.
Some grain dryers utilize two blowers in conjunction with a two plenum grain dryer. These grain dryers also have a heating mode and a cooling mode. One blower is connected with a heating element in order to blow heated air into the upper heating air plenum and through the grain column to heat the grain and remove moisture. The other blower is utilized in conjunction with a lower cooling air plenum. The cooling mode takes ambient air and blows it into the cool air plenum and through the lower half of the grain column, thereby cooling the heated grain.
Heat recovery systems are available on these dual fan/dual plenum grain dryers. These heat recovery systems typically direct the air exiting the cooling portion of the grain dryer into the intake of the blower used in the heating section of the grain dryer The disadvantage to this type of heat recover system is that the cooling air has been partially heated by the hot grain that it is cooling and also removes additional moisture from the grain, thereby decreasing this air's moisture holding capacity below that of the ambient air in the exterior environment of the grain dryer. Therefore, while requiring less energy to heat the air mixture to the desired temperature, the moisture removing capacity of the heated air drying the grain is reduced and the grain flow rate through the dryer will need to be reduced or the amount of air flowing through the dryer increased to compensate for the lower moisture removal capability, resulting in a limited overall savings of energy or time and possibly reducing grain throughput.